Hydrogels can be classified into two broad categories: reversible and irreversible. The former are characterized by significant changes in the rheological properties as a function of temperature, ionic concentration, and dilution. Irreversible hydrogels are soluble in water, as well as in other solvents, over a wide range of temperatures and dilutions. Due to their solubility profiles, irreversible hydrogels have multiple applications in food, cosmetics, medicine and biotechnology [1-5]. The preparation of hydrogels can be achieved by a variety of methods: reticulation of linear polymers; grafting of synthetic polymers onto naturally occurring macromolecules; chelation of polycations; and complexation between polyanions and polycations [6-8].
Chitosan, the deacetylated form of chitin (N-acetyl glucosamide), has polycationic properties and forms, with polyanions, water insoluble hydrogels [9-14]. These have been successfully used as a support material for the immobilization of enzymes and cells [15-20] and for controlled drug release [20-24]. The immobilization of enzymes using organic and inorganic matrices has been widely studied [12,25-29] and a variety of immobilization methods such as chemical reaction, microencapsulation, and adsorption on various surfaces have been used [30-33]. Our attention has focused on enzyme immobilization using hydrogels because their hydrophilicity allows, in principle, for the creation of microsystems favourable for enzymatic activity. Complexes of cationic polymers have been prepared with different polyanions like synthetic polyanions (polyacrylic, polymethacrylic acids and polyvinyl alcohol sulfate [49-51]; natural polyanions (alginate, heparin, carrageenan, pectin, glycosaminoglycanes. [52-54]; cellulosic derivatives (carboxymethylcellulose and oxidized cellulose [58-59]). Complexes of natural polyions such as chitosan, carboxymethylcellulose and alginic acid are already known [8,34,36], and enzymes and other bioactive compounds have been immobilized or lodged therein [37-38]. All the hydrogels known in the art have however a capacity of encapsulation of biological products (medication, enzymes and cells) which retention yield is very low, varying from 1 to 8%. In all these cases, the enzymes conserve only 50% of their initial activity. Therefore, there is clearly a need for polyionic hydrogels showing a more performing retention yield as well as an improved activity for the retained enzymes.